1. Field of the Invention
This invention relates to an electrically rewritable nonvolatile semiconductor memory device and a write/verify method thereof. More specifically, for example, this invention relates to the technique for enhancing the write/verify speed in a flash memory having large memory capacity or a system using the flash memory.
2. Description of the Related Art
The number of memory cells connected to a bit-line is increased with an increase in the memory capacity of a nonvolatile semiconductor memory device and the pitch of the bit lines is narrowed with a reduction in the chip size. Therefore, the capacitance associated with the bit line and the capacitance between the bit lines are steadily increased. The charging/discharging time of the bit line is increased by an increase in the load capacitance of the bit line and time for writing data into a memory cell becomes long. As a result, in a system using the nonvolatile semiconductor memory device, for example, in a system such as a digital camera having a NAND flash memory into which a large amount of data is written, write time will become long.
Further, in the general nonvolatile semiconductor memory device, since it is necessary to verify whether or not data is written to a sufficient extent after the data write operation, longer time is required. The operation of writing data into the NAND flash memory is performed by applying high voltage to the control gate of a cell transistor acting as a memory cell to change (shift) the threshold voltage of the cell transistor. After this, the verify operation is performed to monitor the shifting amount of the threshold voltage of the cell transistor and determine whether or not the shifting amount of the threshold voltage of the cell transistor is sufficiently large. At this time, if the write characteristic of the cell transistor varies, it becomes necessary to increase the number of write and verify operations and it becomes necessary to take further long time to perform the write/verify operation (for example, refer to Jpn. Pat. Appn. KOKAI Publication No. H08-087895).
Recently, a chance of dealing with large capacity data such as moving data, image data of high quality and the like occurs many times and it is desired to enhance the write/verify speed in a flash memory and a system using the flash memory.